An electroabsorption modulator (EAM) is an integrated photonic semiconductor device that allows the intensity of a laser beam to be controlled via an electric voltage. The principle of operation of the modulator is based on applying an electric field to cause a change in the absorption spectrum of the laser without the applied electric field causing an excitation of carriers. A typical EAM has a waveguide and electrodes for applying an electric field in a direction that is perpendicular to the modulated light beam. In order to achieve a high extinction ratio, EAMs typically include a quantum well structure that provides an active region in which carriers are spatially confined such that movement in directions perpendicular to the layers that define the quantum well structure is prevented while movement in directions coplanar with the layer is allowed.
EAMs are capable of operating at relatively low voltages and at very high speeds (e.g., gigahertz (GHz)), which makes them useful for optical fiber communications. A distributed feedback laser (DFB) is a laser in which the entire cavity is made up of a periodic structure that functions as a distributed reflector in the wavelength range of laser action, and contains a gain medium. Typically, the periodic structure contains a phase shift in its middle and is essentially the direct concatenation of two Bragg gratings that provide internal optical gain. An EAM can be integrated with a DFB on a single chip to form an electroabsorption modulated distributed feedback laser (EML) that is capable of operating as a data transmitter.
EML assemblies that operate with low chirp in the 1550 nanometer (nm) range have been proposed for use in, for example, 10 to 40 kilometer (km) optical fiber links for 10 gigabit per second (Gb/s) data rate operations. One difficulty associated with the proposed EML assemblies is that frequency chirp due to back-reflection from the EAM end facet severely limits the propagation span at relatively high data rates (e.g., 10 Gb/s). Thus, minimizing the EAM end facet reflection is needed in order to increase the propagation span.
FIG. 1 illustrates a cross-sectional top view of a known EML assembly 2 comprising a DFB 3 and an EAM 4. The DFB 3 and the EAM 4 are each made up of p-type metal. One end facet 5 of the EML assembly 2 comprises a highly-reflective (HR) or anti-reflective (AR) coating. The other end facet 6 of the EML assembly 2 comprises an AR coating. An inter-contact isolation region 7 electrically isolates the DFB 3 and the EAM 4 from each other. The portions 8A and 8B of the DFB 3 and the EAM 4, respectively, comprise a straight reverse-mesa ridge that extends to, respectively, the end facets 5 and 6. It is the occurrence of back reflection into the EAM 4, and consequently, into the DFB 3, that is originated at the end facet 6 that degrades the performance of the EML assembly 2.
FIG. 2A illustrates a cross-sectional view of the EML assembly 2 shown in FIG. 1 along the A-A′ cross-section shown in FIG. 1. The different semiconductor layers that together comprise the EML assembly 2 are as follows: an n-type (001) Indium Phosphide (InP) substrate 21 having an n-type InP buffer layer 22 formed thereon; a multi quantum well (MQW) active region 23 that is grown on top of the buffer layer 22 by a process known as Selective Area Growth (SAG); a p-type InP spacer layer 25 that is grown on top of the MQW layer 23; a p-type Indium Gallium Arsenide Phosphide (InGaAsP) etch-stop layer 26 that is grown on top of the spacer layer 25; another p-type InP spacer layer 27 that is grown on top of the InGaAsP etch-stop layer 26; a p-type InGaAsP grating layer 28 that is grown on top of the spacer layer 27; the grating layer 28 is selectively etched in the DFB portion 3 to form a periodically varying refractive index region 31 that provides a filter for the laser spectrum in a desired wavelength value; the process then continues with the re-growth of a p-type InP infill 29 and cladding layer 32; and a p-type InGaAs contact layer 33 is then grown on top of the cladding layer 32.
FIG. 2B illustrates a cross-sectional view of the DFB portion 3 of the EML assembly 2 along the E-E′ cross-section shown in FIG. 2A. After the contact layer 33 is grown, a silicon oxide (SiO2) dielectric mask 34 represented by mask portions 34A, 34B and 34C is deposited on the top of the contact layer 33 and a wet chemical etch process is performed to etch the contact layer 33 and the cladding and infill layers 32 and 29. FIG. 3 illustrates a top view of a wafer 38 (the (001) plane, figure not in scale) having the mask 34 shown in FIG. 2B disposed thereon, which comprises straight mask stripes 34A-34C disposed on contact layer 33.
FIG. 4 illustrates an enlarged view of the DFB portion 3 of the EML assembly 2 shown in FIGS. 2A and 2B along the E-E′ cross-section shown in FIG. 2A after the chemical etching process has been performed. When the chemical etch process is performed, the device 2 etches relatively quickly in the (001) plane, but relatively slowly in the (111) plane, causing ridges 35 to be formed with lateral (111) facets 36 thereon, having an angle, ω, between the (001) and (111) facets that is typically about 54.7 degrees. This ridge configuration is commonly referred to as a reverse-mesa configuration due to the fact that the top of the ridge is wider than the base of the ridge, whereas the usual understanding of a mesa is a formation having a generally flat top surface and sides that extend down to a base that is at least as large as the top surface area.
In order for the EML assembly having the reverse-mesa configuration shown in FIG. 4 to perform well, the base width Wn of the ridge structure 35 must be determined with high precision. In fact, this dimension determines, in conjunction with the grating characteristics, the spectral behaviour of the DFB 3 and the modulating properties of the EAM 4. This means that the height H (i.e., the distance from the top surface of layer 31/28 to the top surface of layer 33) and the widths, WA, WB and WC, of the respective mask portions 34A, 34B and 34C also must be very precise. This is especially true for the width WB. Thus, the entire process comprising the growths of the infill 29, cladding 32 and contact 33 layers, the deposition of mask 34 and the wet chemical etching process must be precisely controlled. This requirement for precision is very close to or beyond the technical limits of the techniques adopted in the EML fabrication industry (i.e., growth control, film deposition kits currently available for creating masks, photolithography, wet chemical etching solutions), which leads to the ridge structures 35 having the facets 36, and consequently the width Wn, not being as precise as is necessary to provide the correct dimensions of the ridge structures needed for optimal operation of the EML device. This imprecision in the definition of the ridge structures is the main factor affecting the fabrication yield for EML assemblies of this type.
The requirement of having very low reflectivity at the EAM end facet 6 (FIG. 1) can be relaxed somewhat by forming a bent or tilted waveguide structure (i.e., a bent or tilted reverse-mesa ridge structure configuration) at the distal end portion of the EAM portion 4 adjacent to the EAM end facet 6. An attempt to make such a structure using the process described above with reference to the straight ridge configuration of FIGS. 2A-4 will be affected by the same technological difficulties previously described, i.e., the inability to precisely control the height H and width of the masks 34A-34C will lead to the wet chemical etch process producing imprecise facets 36 and imprecise width Wn (FIG. 4). Furthermore, the width WB must vary along the bent section in order to ensure that the width of the ridge structure is kept constant. This imprecision would result in poor performance of the EML assembly and, even more importantly, in a lower process yield. Consequently, it is very difficult to produce EML assemblies with straight, bent or tilted ridge structures with high yield at low cost using this process.
Dry etching techniques have been used to create ridge structures that are tilted with respect to the main crystallographic axis of the wafer. For example, U.S. Pat. No. 6,542,533 discloses the use of a dry etching technique known as reactive ion etching (RIE) to form ridge structures. However, there are problems associated with using a dry etching technique to form the ridge structures. It is difficult to control the etch depth when using dry etching, which leads to difficulties in obtaining precise and reproducible control over the etch depth. In addition, dry etching typically leads to the ridge structure having a rectangular shape. When there are limits on the ridge width, such as in cases where monomodal propagation is required, the rectangular shape makes it difficult to achieve low serial resistance in the device. Another problem is that dry etching results in crystal damage and passivation of p-type doping species, which makes it necessary to perform an annealing process after the etching process has been performed. In addition, performing the annealing process does not lead to a complete recovery from the crystal damage and passivation.
Accordingly, a need exists for a way to create EML assemblies having straight ridge structures that are more precisely formed in order to provide improved performance and higher manufacturing yield. A need also exists for a way to create EML assemblies and other devices with bent or tilted ridge structures to reduce the requirement of low reflectivity at the EAM end facet, thereby providing improved performance and higher manufacturing yield.